The present disclosure relates to apparatus and methods for impedance measurement and, more specifically, to impedance measurement in energy storage cells employed in rechargeable service, as well as systems including such cells. Specific applications, without limitation, include impedance measurement of relatively high voltage energy storage cells.
Chemical changes to electrodes in a rechargeable battery may cause degradation in the battery's capacity, and other functional parameters. Battery degradation may accumulate over the life of the battery. Environmental factors (e.g., high temperature) and functional factors (e.g., improper charging and discharging) may accelerate battery degradation. Operators of systems that rely on rechargeable battery power may desire to monitor the degradation of the batteries they use.
One indicator of battery degradation is an increase in battery impedance. FIG. 1 is an impedance (real and imaginary) plot 102 (similar to a Nyquist plot) of a fresh battery and an impedance plot 104 of an aged battery, measured at several different frequencies using an Electrochemical Impedance Measurement (EIM) system. As illustrated in FIG. 1, the aged battery shows a higher impedance than the fresh battery at each of the different frequencies. Operators of systems that rely on rechargeable batteries may use impedance data, such as the EIM data of FIG. 1, to determine that a replacement battery is needed before a failure occurs. Such preemptive replacement may prevent expensive delays and property damage that may occur in the event of a battery failure. Also, knowledge of a battery's continued reliability may prevent expenses associated with unnecessarily replacing a battery that still has a substantial amount of lifetime left.
EIM systems use the Bode analysis technique to characterize an impedance of an electrochemical process. The Bode analysis technique is a well-established and proven technique. In EIM systems, a battery being evaluated is excited with an AC current of a single frequency, and a response is measured. The process is repeated over a range of frequencies of interest until the spectrum of the impedance is obtained. The EIM method is effective, but time consuming, as the process is serial (e.g., the impedance is measured separately and sequentially for each of the different frequencies of interest).
A parallel approach using bandwidth limited noise as an excitation current to the battery can obtain similar impedance information of the battery in less time. System response to the noise is processed via correlation and Fast Fourier Transform (FFT) algorithms, and many such responses are averaged. The result is the spectrum of response over the desired frequency range. The averaging of many responses makes this process somewhat serial (e.g., separate excitations are sequentially applied to the battery, and respective responses are measured). As a result, this process is also somewhat time consuming, similar to the EIM system discussed above. Another technique assembles the current noise waveform from a sum of sinusoids, each at a different frequency. The system response as a time record is acquired and processed with the FFT algorithm. To reduce noise, multiple time records of waveforms are processed and their resultant spectra are averaged. This process is similarly somewhat serial, and therefore, is also somewhat time consuming.